1. Field of the Invention
The present invention relates to planar optical waveguides, and particularly to lithographically formed, tapered waveguides connectable to optical fibers.
2. Technical Background
Optical waveguide structures are used to build integrated optical circuits that route and control optical signals in a optical fiber communication system. In optical communication systems, messages are transmitted at infrared optical frequencies by carrier waves that are generated using sources such as lasers and light-emitting diodes. There is interest in these optical communication systems because they offer several advantages over electronic communications systems using copper wires or coaxial cable. They have a greatly increased number of channels of communication, as well as the ability to transmit messages at much higher speeds than electronic systems.
The operation of an optical waveguide is based on the fact that when a core medium which is transparent to light is surrounded or otherwise bounded by another cladding medium having a lower refractive index, light introduced along the core medium""s axis is highly reflected at the boundary with the surrounding cladding medium, thus producing a light-guiding effect.
One method used to form an optical waveguide device involves the use of standard photolithographic processes. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer-containing layer deposited on a substrate. This layer may itself consist of several layers composed of the same or different polymeric materials having dissimilar refractive indices, to form a core, overcladding, and undercladding layers or structures.
Passive polymer optical waveguides are used to form interconnections between optical information processing devices or interconnections between such devices and other external optical communication links such as glass optical fibers. They may also be used to create passive optical devices such as bends, splitters, combiners, couplers, and the like. In some instances, it is desired that these waveguiding structures have varying dimensions in different directions in order to efficiently intercept other optical elements or have a certain beam profile. Such may be done by tapering the waveguide. This tapering, especially when thickness variation is desired, is not obtainable with conventional mask fabrication techniques where exposure power from a large area UV lamp is constant, and the incident light is unfocused. However, using direct writing lasers, the focal plane and the intensity of the laser beam as well as the scanning speed of translating a target relative to the laser beam, can be adjusted to produce tapered structures in a single step.
In order to connect waveguides to optical fibers, a pigtailing technique is required. Pigtailing refers to attaching one light carrier to another light carrier, such as attaching a waveguide to an optical fiber. Pigtailing is especially difficult with single-mode structures where a small (1-2 xcexcm) misalignment can result in serious coupling loss. This may be due to the small dimensions involved or the fact that the core of a single-mode fiber can be displaced from the exact center of the fiber. Typically, these problems are circumvented by active fiber alignment where an operator displaces the fiber relative to the waveguide while monitoring the power output and attaches the fiber when maximum power is detected. This typical process is time consuming and expensive. The present invention provides a technique for pigtailing waveguides with relaxed tolerances. This is done by expanding the height and width of a waveguide which allows a relaxation of the tolerance requirement.
The invention produces varying dimension tapered polymer waveguides by laser direct writing in a uniform thickness photopolymerizable layer. Photopolymerizable compositions are direct written by a laser beam while varying the exposure dose directed to the composition. This may be either by controlling the laser power or by controlling the scanning speed of a target substrate with respect to the laser beam. Focusing the laser beam on the substrate surface allows one to grow waveguides from the bottom up as the substrate is scanned or the laser power is varied. This bottom up polymerization is obtained despite the fact that the exposure is from the direction of the top of the layer. This is done by focusing of the laser beam at the bottom of the polymerizable layer where the beam has a higher power density than at the top. This result is also due to self-guiding effects, since as the laser beam travels through the polymerizable layer, it changes the index of refraction of the material in its path. This results in yet sharper focusing of the beam, resulting in further increases in the power density at the bottom of the layer. Thus one can produce a waveguide having any desired thickness from zero through the total thickness of the starting unexposed layer.
The invention provides a process for forming a tapered waveguide on a substrate. An actinic radiation polymerizable coating composition is applied on a substrate. The coating composition has a bottom surface adjacent to the substrate and a top surface spaced from the bottom surface and the coating composition is at least partially transparent to laser light. The coating composition is imagewise exposed, in an oxygen containing atmosphere, to sufficient actinic radiation to at least partially polymerize the coating composition and form a polymerized portion and a non-polymerized portion of the coating composition by directing a converging beam of laser light onto and through the coating composition. The beam of converging laser light has a greater intensity at the bottom surface of the coating composition, a lesser intensity at the top surface of the coating composition, and a gradually decreasing intensity from the bottom surface of the coating composition to the top surface of the coating composition. Then either the substrate is moved with respect to the converging beam or the converging beam is moved with respect to the substrate along a linear path, or a combination may be used. One then either conducts the moving at a gradually increasing or gradually decreasing velocity from a first position on the substrate to a second position on the substrate; or gradually increases or decreases the intensity of the beam of laser light from a first position on the substrate to a second position on the substrate; or a combination of the two may be used. After developing the coating composition with a liquid developer and removing the non-polymerized portion of the coating composition a tapered waveguide is formed on the substrate.
The invention also provides a process for attaching an optical fiber having a core with a first cross sectional area to an optical fiber or waveguide having a core with a second cross sectional area which comprises forming a tapered waveguide on a substrate by first providing an actinic radiation polymerizable coating composition on a substrate. The coating composition has a bottom surface adjacent to the substrate and a top surface spaced from the bottom surface and the coating composition is at least partially transparent to laser light. One then imagewise exposes, the coating composition, in an oxygen containing atmosphere, to sufficient actinic radiation to at least partially polymerize the coating composition and form a polymerized portion and a non-polymerized portion of the coating composition by directing a converging beam of laser light onto and through tbe coating composition. The beam of converging laser light has a greater intensity at the bottom surface of the coating composition and a lesser intensity at the top surface of the coating composition, and a gradually decreasing intensity from the bottom surface of the coating composition to the top surface of the coating composition. One then either moves the substrate with respect to the converging beam or moves the converging beam with respect to the substrate along a linear path; or a combination may be used. One either conducts the moving at a gradually increasing or gradually decreasing velocity from a first position on the substrate to a second position on the substrate; or one gradually increases or decreases the intensity of the beam of laser light from a first position on the substrate to a second position on the substrate; or a combination of the two may be used. Afier developing the coating composition with a liquid developer and removing the non-polyrnerized portion of the coating composition forming a tapered waveguide is formed on the substrate. The waveguide has a first end having a first cross sectional area and a second end having a second cross sectional area. Then at least one optical fiber is attached to at least one end of the waveguide.